Electrical resistance welder for nails used in an automatic nailing machine

ABSTRACT

In an electrical resistance welder, whenever a material to be welded approaches the welding electrodes, a start signal is produced, so that welding current is supplied to the welding transformer in one direction for a certain period of time set by a time limit circuit, with the polarity changing in response to the start signal, whereby, even if materials are repeatedly fed for welding at high speed, they can be positively welded in a uniform manner.

BACKGROUND OF THE INVENTION

This invention relates to electrical resistance welders, and moreparticularly to an electrical resistance welder which is suitable forwelding relatively small materials.

In welding small materials such as thin metal wires, it is necessary toapply a relatively small current for a short time. In such a case,heretofore a capacitor type welder is employed. However, the welder isdisadvantageous in that it takes a relatively long time to charge thecapacitor of the welder, and it is impossible to increase the weldingrate because of the service life of the capacitor.

In order to overcome this difficulty, a welding method has been proposedin which the half cycles of a commercial main frequency are utilized forwelding. However, it is difficult to correctly supply current to thematerials which are repeatedly fed for welding at high speedindependently of the phase of the AC power source.

SUMMARY OF THE INVENTION

Accordingly, an object of this invention is to provide an electricalresistance welder which can suitably supply current to materials whichare repeatedly fed for welding at high speed.

The foregoing object and other objects of the invention have beenachieved by the provision of an electrical resistance welder which,according to the invention, comprises: a bridge type inverter made up oftransistors; a welding transformer connected to the AC output side ofthe bridge type inverter; the first time limit circuit which is startedby a start signal; a flip-flop whose state is changed whenever the startsignal is supplied thereto; the second time limit circuit which isstarted by output of the first time limit circuit; and two gate circuitswhich receive the outputs of the flip-flop and the second time limitcircuit, so that the welding current is supplied to the weldingtransformer for a predetermined time interval provided by the time limitcircuits with its polarity changing alternately whenever the startsignal is provided.

The nature, principle and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a circuit diagram, partly as a block diagram, showing oneexample of an electrical resistance welder according to this invention;

FIG. 2 is an explanatory diagram showing the essential components of theelectrical resistance welder according to the invention; and

FIG. 3 is a plan view of an article which is manufactured by the welderof the invention.

DETAILED DESCRIPTION OF THE INVENTION

One preferred embodiment of this invention shown in the accompanyingdrawings is an electrical resistance welder which is used for nails foran automatic nailing machine. The nails are spot-welded to two thinparallel metal lines at predetermined intervals as shown in FIG. 3.

In FIG. 1, reference numerals 1 and 2 designate one set ofseries-connected transistors; 3 and 4, another set of series-connectedtransistors, these sets of transistors being connected to a powersource; 5, a welding transformer whose primary winding is connected tothe connecting point of the transistors 1 and 2 and to the connectingpoint of the transistors 3 and 4, the secondary winding of the weldingtransformer 5 being connected to a welding electrodes; 6, a drivecircuit for driving the transistors 1 and 4; 7, a drive circuit fordriving the transistors 2 and 3; 8 and 9, gates whose outputs aresupplied to the drive circuits 6 and 7, respectively; and 10, aflip-flop which supplies an enabling signal to the gate 8 when reset andapplies an enabling signal to the gate 9 when set. A position detector29 supplies a start signal to the flip-flop 10. The state of theflip-flop 10 is changed whenever the start signal is supplied to theflip-flop 10.

Further in FIG. 1, reference numerals 11 and 12 designate time limitcircuits. The second time limit circuit 11 is started by the output ofthe first time limit circuit 12. The first time limit circuit 12 outputsa signal each time the start signal is applied by the position detector29. The output of the second time limit circuit 11 is supplied to thegates 8 and 9, so that it is supplied to one of the drive circuits 6 and7 depending on the state of the flip-flop 10.

Referring to FIG. 1, reference numerals 13 through 16 designate diodeswhich are parallel-connected opposite in polarity to the transistors 1through 4; 17, a rectifier circuit adapted to rectify the alternatecurrent of an AC power source 18 thereby to feed a direct current to abridge type inverter consisting of the aforementioned transistors 1through 4; 19, a capacitor; 20, a transistor provided between therectifier circuit 17 and the transistor inverter, to operate to maintainthe voltage of the capacitor 19 constant with the aid of a controlcircuit 21; and 22, a current limit resistor.

In FIG. 2, reference numeral 23 simply designates a welding currentcircuit which is a whole of the circuitry shown in FIG. 1; and 24, adisk having grooves cut in the periphery, the grooves being used to holdnails. The disk 24 is turned by an electric motor (not shown), and nailsa are supplied into the grooves 25 from a chute 26.

Further in FIG. 2, reference numeral 27 designates a wire feeding guideprovided close to the disk 24, to feed wires b along the array of nailsin the grooves 25; 28 and 28, roller electrodes; and 29, theaforementioned position detector which is mounted on the shaft of thedisk 24. The roller electrodes 28 operate to push the wires b againstthe nails a in the grooves 25 of the disk 24 with the aid of apressurizing mechanism (not shown). The roller electrodes 28 areelectrically insulated from each other and are connected to thesecondary winding of the welding transformer 5. The position detector 29produces a signal immediately before each groove 25 reaches the rollerelectrodes 28 by the rotation of the disk 24. This signal is supplied tothe time limit circuit 12 and the flip-flop 10 as the aforementionedstart signal.

In operation, the disk 24 is rotated, while receiving the nails a.Therefore, the nails a together with the wires b approach the electrodes28. Whenever each nail a comes near the electrodes 28, the positiondetector 29 provides the signal, as described before. The nail a reachesthe roller electrodes 28 during the time limit period of the time limitcircuit 12. Thereupon, the time limit circuit 11 is started. When theflip-flop 10 is in the reset state, the output of the time limit circuit11 is applied through the gate 8 to the drive circuit 6, as a result ofwhich the transistors 1 and 4 are maintained conductive (ON) for thetime interval determined by the time limit circuit 11. Accordingly, apulse current is supplied through the transistors 1 and 4 to the primarywinding of the welding transformer 5. Owing to the primary current, thesecondary current flows from one of the roller electrodes 28 through thewire b, the nail a and the other wire b to the other roller electrode28, as a result of which the nail a is welded to the wires b.

As the disk 24 is further rotated, the next nail a approaches the rollerelectrodes 28, whereupon the position detector produces the signal againand therefore the flip-flop 10 is set. The time limit circuits 11 and 12operate similarly as in the above-described case. The output of the timelimit circuit 11 is supplied through the gate 9 to the drive circuit 7,so that the transistors 2 and 3 are rendered conductive. Therefore, thesame pulse current is supplied in the opposite direction to the primarywinding of the welding transformer 5 through the transistors 2 and 3.Owing to the primary current, the secondary current flows in theopposite direction, as a result of which the nail is welded to thewires.

As is apparent from the above description, according to the invention,the current is supplied to the primary winding of the weldingtransformer alternately in the opposite directions by one time limitcircuit for the predetermined period of time. Accordingly, thetransformer is never given the direct current component of excitation.The energization time can be changed readily. Furthermore, the currentvalue can be readily controlled by changing the turn ratio of thetransformer, or the supply voltage. The energization can be started atany time. Therefore, no matter when materials to be welded are supplied,welding can be carried out under predetermined conditions.

In the above-described embodiment, the diodes 13 through 16 areparallel-connected opposite in polarity to the transistors 1 through 4.This is for feedback to the power source in the case where the reactanceof the secondary circuit of the welding transformer 5 is large. Thetransistor 20 and the control circuit 21 are provided to maintain thewelding current constant even when the supply voltage varies. Thesecondary circuit shown in this embodiment is such that two weldingpoints are in series between the roller electrodes 28; however, thecurrent may be supplied between the disk 24 and the roller electrodes28. The fine adjust of the relative position of the disk 24 and theposition detector 29 can be replaced by controlling the limit time ofthe time limit circuit 12.

The transistors 1 through 4 may be connected in series or in paralleldepending on the capacities thereof, and a snubber circuit comprisingresistors and capacitors may be added.

What is claimed is:
 1. An electrical resistance welder for welding nailsused in an automatic nailing machine comprising:a rotary disk havinggrooves cut in a periphery thereof at predetermined intervals forholding said nails; means for successively feeding nails into saidgrooves; means for rotating said disk to successively place said nailsin a predetermined welding position; means for feeding a pair of wiresat a constant speed to said position where said wires can be contactedwith one of said nails held in said grooves; a position detector forproducing a signal upon each rotation of said disk by a predeterminedrotating angle corresponding to said intervals; and energization meanshaving current supplying elements contacting said wires, for supplyingmomentary currents through a circuit consisting of said wires and a nailcontacted therewith at said welding position each time said signal isproduced; said rotary disk, energization means and feeding meanscooperating to weld nails across said pair of wires successively so asto make nails suitable for use in an automatic nailing machine.
 2. Anelectrical resistance welder as claimed in claim 1 wherein saidenergization means comprises:a bridge type inverter made up oftransistors; a welding transformer connected to the AC output side ofsaid bridge type inverter; a first time limit circuit which is startedby a signal from said position detector; a flip-flop whose state ischanged whenever said signal is supplied thereto; a second time limitcircuit which is started by the output of said first time limit circuit;and two gate circuits which receive outputs from said flip-flop and saidsecond time limit circuit; and, first and second drive meansrespectively connected to the outputs of said gate circuits, each ofsaid drive means respectively driving transistor pairs of said bridgecircuit, so that current is applied to said welding transformer for apredetermined limit time provided by said second time limit circuit withthe polarity thereof changing alternately whenever said start signal isproduced.